Method and apparatus for producing a tube of glass

ABSTRACT

A method and apparatus are provided for producing a tube of glass by zonewise heating and softening of a hollow cylinder by a movable heating zone while rotating about its rotation axis. The glass tube is continuously formed by radial expansion of the softened region under action of centrifugal force and/or internal overpressure applied in the hollow-cylinder bore. The method and apparatus make it possible to deform the hollow cylinder in a single or a small number of forming steps into a glass tube having a larger outer diameter and high dimensional accuracy by determining a circumferential position at which the wall thickness is comparatively small, and during heating and softening of the rotating hollow cylinder a coolant is dispensed from a coolant source onto the deformation zone only when or predominantly when the circumferential position having the comparatively small wall thickness passes the coolant source.

BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to a method for producing a tube of glass,particularly of quartz glass, comprising the following method steps:

-   -   (a) providing a hollow cylinder of the glass having a wall        thickness and having an outer diameter D₁,    -   (b) zonewise heating and softening the hollow cylinder which is        rotating about a rotation axis in a heating zone moved relative        to the rotation axis,    -   (c) forming a deformation zone by radial expansion of the        softened area under the action of a centrifugal force and/or an        internal overpressure applied in the hollow cylinder bore, and    -   (d) continuously forming the tube to produce an outer diameter        D₂ which is greater than D₁.

Moreover, the present invention relates to an apparatus for performingsuch a method, comprising:

-   -   a rotation device for rotating a hollow cylinder of glass about        its longitudinal axis, which cylinder has an inner diameter, an        outer diameter D₁ and an inner bore defined by a wall, and    -   a heater which is movable relative to the hollow cylinder for        the zonewise heating and softening of the hollow cylinder and        for forming a tube having an outer diameter D₂ which is greater        than D₁.

With the help of such methods and apparatuses, hollow cylinders ofglass, particularly of quartz glass, are formed in one or several hotforming steps into tubes having an increased outer diameter. An initialhollow cylinder which is rotating about its longitudinal axis is heresoftened zone by zone in a heating zone, which is moved at a relativefeed rate in relation to the hollow cylinder, and is expanded in thisprocess under the action of a radially outwardly directed force eitheragainst a molding tool arranged at a given radial distance from thelongitudinal axis of the tube, or it is formed without tools. Theradially outwardly directed force is based on the centrifugal forceand/or on an internal overpressure in the inner bore of the hollowcylinder (also called “blow pressure”).

To observe the dimensional accuracy of the drawn-off tube strand, atleast one of its dimensions, for example outer diameter, inner diameter,or wall thickness, is controlled. The blow pressure, the relative feedrate between hollow cylinder and heating zone, and the temperature inthe heating zone are common manipulated variables of the control.

Prior Art

The larger the tube end diameter, the more difficult and expensivebecomes the production of a dimensionally accurate large tube. Tomitigate these problems, Japanese patent application publication JP2004-149325 A suggests that the forming process should be subdividedinto a plurality of forming steps with successive increase in diameter.To this end the hollow cylinder of quartz glass to be formed, having adiameter of 250 mm, is clamped in a lathe and rotated about itshorizontally oriented longitudinal axis while being heated by an annulararrangement of heating burners and is thereby softened zone by zone,wherein the heating burners are moved at a given feed rate along thecylinder jacket. The increase in diameter is due to the centrifugalforce acting on the softened region. The deformation zone will travelonce along the entire initial cylinder until the cylinder is completelyexpanded. The outer diameter of the tube is here sensed continuouslywithout tools by a laser beam. This forming step will be repeated untilthe nominal tube diameter of 440 mm is reached. In each forming step thetube diameter is increased by 15 mm.

In this forming process, one achieves a comparatively small formingdegree in each individual forming step, which is accompanied by areduced deviation from the target value of a radial tube dimension.Moreover, in each forming step it is possible to take into account andcorrect dimensional deviations existing in the respective initialcylinder.

On the other hand, it is evident that this procedure is very time- andenergy-consuming, especially since the tube cools down betweensuccessive forming steps.

European patent application publication EP 0 037 648 A1 describes amethod of producing optical fibers in which a tube is formed by zonewiseheating and application of an internal overpressure into a tube havingan increased inner diameter.

U.S. Pat. No. 5,167,420 describes an apparatus for producing asurrounding groove in a glass tube, wherein the viscosity of the glassis reduced in the area of the groove by active cooling.

Japanese patent application publication JP H10-101353 A describes amethod for producing a quartz glass tube, wherein a quartz glasscylinder is softened zone by zone by applying an internal overpressureand is formed while rotating about its longitudinal axis against anouter molding tool into the tube. The quartz glass cylinder is hereclosed at one side. Besides the outer diameter, it is the aim to achievea uniform wall thickness. To this end parallel mold plates are used onthe molding tool.

German Patent DE 41 21 611 C1 describes a method for producing quartzglass tubes, in which the wall thickness of the drawn-off quartz-glasstube strand is regulated. A hollow cylinder of quartz glass is herepushed continuously while rotating through a heating furnace withinwhich water-cooled graphite plates are arranged at a radial distancefrom the longitudinal axis of the tube. Due to overpressure within thehollow cylinder the soft hollow cylinder is blown against the graphiteplates, so that the radial distance of the graphite plates from thelongitudinal axis of the tube roughly predetermines the resulting outerdiameter of the tube. Viewed in the feed direction of the blank relativeto the furnace, soft quartz glass accumulates in front of the graphiteplates and forms a circumferential bead around the outer wall of theblank. It is suggested that the height of the circumferential beadshould be used for process control by optically sensing the bead heightby a camera and by using the deviation from a predetermined target beadheight for process control. The overpressure in the inner bore of thehollow cylinder is chosen as a manipulated variable of the control.Variations of the inner diameter of the tube and thus variations of thewall thickness of the tube can thereby be minimized.

Technical Object

It may be tried to keep the number of forming steps as small aspossible, wherein the respective deformation degree, i.e. the change indiameter, is set to be as high as possible. However, it has been foundthat dimensional deviations already existing in the original hollowcylinder tend to continue into the drawn-off glass tube in the formingprocess and are even intensified. Variations in the radialcross-sectional profile or wall one-sidedness, i.e. radially irregularcourse of the tube wall thickness, which is also called “siding” amongthe experts, are particularly disadvantageously noticeable. Since theouter diameter is a relatively fixedly predetermined value in the use ofa molding tool, tube wall siding is in this case accompanied byfluctuations in the inner diameter of the tube.

With increasing tube end diameter, these problems increase. The reasonis that in the forming process wall thickness variations, which arefound in the initial cylinder, exponentially rise with the diameter.Therefore, the maximum values for siding (e.g. 1 mm), which are stilltolerable according to the specification, may in the final analysislimit the tube end diameter that can be realized in practice.Comparatively thin wall areas of the hollow cylinder deform more easilythan rather thick-walled areas. The greater the blow pressure, the morethe thickness difference will be noticed, so that the blow pressurecannot be arbitrarily high. Instead, in order to achieve commerciallyacceptable forming rates, the glass must be heated at a highertemperature and softened more strongly. This, however, results inpronounced drawing streaks and other defects in the glass wall and in anincreased energy demand, especially in the case of large-volume tubes(hereinafter also called “large tubes”), which on account of their largesize cool down particularly rapidly.

BRIEF SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide a method whichmakes it possible to form a hollow cylinder in a single forming step orin a small number of forming steps, if possible, into a glass tubehaving a large outer diameter and high dimensional accuracy.

Moreover, it is the object of the present invention to provide anapparatus suited for performing the method.

As for the method this object, starting from a method of the typementioned at the outset, is achieved according to the invention,wherein:

(i) providing the hollow cylinder according to method step (a) comprisesthe determination of a circumferential position at which the wallthickness is comparatively small, and

(ii) during heating and softening of the rotating hollow cylinder acoolant is dispensed from a coolant source onto the deformation zoneonly when or predominantly when the circumferential position having thecomparatively small wall thickness passes the coolant source.

In the method according to an embodiment of the invention at least onecircumferential position is determined having a comparatively small wallthickness along the longitudinal axis of the hollow cylinder. The smallwall thickness is for instance the minimal wall thickness in a radialcross-section around the circumference of the hollow cylinder. Thecircumferential position of the small wall thickness shall also becalled “thin wall point” hereinafter, for short.

For the determination of the thin wall point in the sense of the abovemethod step (i) the wall thickness profile around the circumference isdetermined directly or indirectly. With an indirect determination, forinstance, the measurement of the inner diameter alone is enough when theouter diameter of the hollow cylinder can be assumed to be constant.This determination can be carried out in a separate measurement processin advance with respect to the forming process, or the determination iscarried out successively during the forming process, but preferablybefore the length section of the hollow cylinder in question passes intothe heating and deformation zone. The circumferential position of thethin wall point may shift, viewed over the length of the hollow cylinderto be formed. As a rule and in the simplest case, however, it is thesame over the entire length of the hollow cylinder. Under thisprecondition the measurement of a hollow-cylinder ring is enough to beable to determine the circumferential position in question in the senseof the above method step (i) over the entire length of the hollowcylinder.

According to method step (ii) a coolant acts on the thin wall point asit passes into the area of the heating zone and thus into thedeformation zone. The action of the coolant is periodic, i.e., it onlyhappens if due to the rotation of the hollow cylinder the thin wallpoint passes a source, which is stationary in circumferential direction,for the discharge of coolant. The action may take place after eachpassage of the thin wall point or after a predetermined number ofpassages. With a controlled action the coolant is dispensed with apassage of the thin wall point in response to a cooling degree which isto be observed and is predetermined. At any rate, the discharge ofcoolant is changed in time more or less regularly in response to theclock frequency of the hollow-cylinder rotation, which will also becalled “periodic coolant discharge” hereinafter.

The area of action to which the discharge of coolant is restricted canbe regarded as a circular arc or circumferential section around thedeformation zone. Coolant will be dispensed only if or predominantlywhen the circumferential section or circular arc is located in thecoolant discharge area. When the thin wall point leaves that area, thedischarge of coolant will be terminated or reduced. A maximallyintensive cooling of a thin wall point as compared with the remainingwall point will be achieved if upon each rotation of the hollow cylinderabout its longitudinal axis the coolant discharge is activated exactlyonce and then deactivated again completely.

Due to the action of the coolant the viscosity of the glass is locallyincreased around the thin wall point, whereby the deformability of theglass mass is reduced in this section of the deformation zone. The thinwall point is thus less strongly deformed; hence, it remains thickerthan would be the case without the cooling locally acting thereon.Attention must here be paid that also a selective coolant discharge ontothe thin wall point effects not only an increase in viscosity at exactlythe circumferential position of the thinnest wall thickness, but also inneighboring areas—though to a lesser degree. The viscosity of the glassshows exponential temperature dependence, so that temperature variationsof a few degrees already have a noticeable impact on viscosity.

Owing to the periodic coolant discharge, the viscosity of the glass massrotating about the longitudinal axis of the hollow cylinder isinfluenced over the circumference of the deformation zone such that amore uniform preforming in the deformation zone is achievedindependently of the wall thickness profile of the hollow cylinder. Evenwith a periodic coolant discharge, the discharge amount of coolant ineach period and thus the intensity of the cooling action can be setselectively in response to a radial tube or hollow-cylinder dimension,and can be used specifically as a manipulated variable of a control forthe tube or hollow-cylinder dimension in question. This dimension isparticularly the wall thickness or the inner diameter.

“Deformation zone” is understood as that area in which the glass mass isplastically deformable and in which the geometry of the drawn-off tubecan be influenced by cooling. In the deformation zone the outer diameteris continuously increasing from the hollow cylinder to the tube, and thewall thickness is normally decreasing, but it may also remain about thesame.

The “beginning” of the deformation zone is defined as that x-position(along the longitudinal axis of the hollow cylinder) at which thefollowing applies to the location-dependent outer diameter D_(V) of thedeformation zone: D_(v1)=D₁+( 1/10)·(D₂−D₁). Likewise, the “end” of thedeformation zone marks that x-position where the following applies tothe location-dependent outer diameter D_(V) of the deformation zone:D_(v2)=D₂−( 1/10)·(D₂−D₁).

The method according to the invention reduces the unfavorable effect ofexisting wall one-sidedness of the hollow cylinder and thereby allowscomparatively large diameter variations in one or each forming stage.This allows an economic forming process having few forming steps.Ideally, only one single forming step is required. Specifically, it isthereby possible to produce large tubes of quartz glass having outerdiameters of more than 500 mm with acceptable energy expenditure andwithout pronounced drawing streaks and tolerable siding.

It has proven to be particularly advantageous when a liquid,particularly water, is used as the coolant.

Thermal energy is removed from the deformation zone during evaporationof the liquid. Preferably, water is used which is distinguished by aparticularly high evaporation enthalpy and evaporates without residuesfrom the surface of the deformation zone. In this respect the use ofdeionized water has proven to be particularly advantageous.

It has proven to be useful when the liquid is sprayed or splashed ontothe deformation zone.

The spraying of the liquid in the form of fine droplets and thesplashing in the form of a liquid jet allow a direct, locally definedapplication of the liquid, particularly of water. A small liquid amountis enough. It can be supplied rapidly as soon as the thin wall pointenters the intended region of the coolant discharge, and it can berapidly deactivated or reduced as soon as the thin wall point leavesagain the intended region of the coolant discharge.

Preferably, the amount of liquid is changed in the cycle of thehollow-cylinder rotation.

The amount of cooling liquid dispensed by the coolant source varies inthe cycle of the hollow-cylinder rotation; this means that it changesupon each rotation of the hollow cylinder at least twice. After the thinwall point has entered into the area of the coolant discharge, thecoolant discharge is activated or increased, or it is deactivated ordecreased as soon as the thin wall point leaves the intended area of thecoolant discharge again.

For the compensation of a plurality of circumferentially distributedthin wall points, the coolant source can be activated several timesduring a rotation of the hollow cylinder. A particularly efficientcooling of a thin wall point as compared with the remaining wallcircumference will however be achieved if the discharge of the coolantis activated exactly once and is deactivated completely exactly onceduring each rotation of the hollow cylinder about its longitudinal axis.

The shorter the circular arc around the deformation zone in which thecoolant acts on it, the more efficiently can the viscosity be increasedexactly in the area of the thin wall point. In this respect, it isprovided in a preferred method embodiment that a circular arc around thedeformation zone in which the coolant is operative is smaller than 30angular degrees.

When high demands are made on dimensional accuracy and processstability, a procedure is preferred in which the internal pressure isset to less than 20 mbar, preferably to less than 10 mbar.

It has been found that a high internal pressure (blow pressure) canimpair the process stability. The tangential tension which is operativein the tube wall due to the blow pressure depends on the wall thickness.The thinner the wall, the more noticeable is the internal pressure onthe deformation in the deformation zone in tangential direction. Thishas the effect that wall thickness deviations existing in the hollowcylinder are intensified in the deformation zone under the action ofblow pressure, because a thinner wall is subject to a higher tangentialtension than a thicker wall.

With the formerly known forming methods, diameter changes (D₂−D₁) ofmore than 40 mm were hardly possible without toleration of formingerrors in the forming of hollow cylinders of quartz glass. Such diameterchanges can be managed without any problems with the method according toembodiments of the invention. Even with diameter changes of 120 mm in asingle forming stage, no inhomogeneities were observed in the drawn-offtube strand or instabilities in the process sequence.

Hence, in the method according to embodiments of the invention largediameter changes are preferred, so that the tube is produced having anouter diameter D₂ which is greater by at least 40 mm, preferably by atleast 70 mm and particularly preferably by at least 100 mm than D₁.

It is thereby possible to set a diameter change of 40 mm or more in asingle forming stage, preferably more than 70 mm and particularlypreferably more than 100 mm, so that a particularly economic formingmethod having a few forming steps is also possible in the case of largediameter changes. Ideally, only one single forming step is required. Itis thereby particularly possible to produce large tubes of quartz glasshaving outer diameters of more than 500 mm with acceptable energyexpenditure and without pronounced drawing streaks and tolerable siding.

It is intended in a particularly preferred method embodiment that thetemperature profile around the circumference of the deformation zone bedetermined.

The coolant periodically acting on the thin wall point locally cools thesurface in the area of the deformation zone. By measurement of thesurface temperature around the circumference of the deformation zone oneobtains information about the degree of cooling in relation to the leveland the local distribution of the temperature. The cooling measure canbe adapted or controlled on the basis of this temperature measurement inthat cooling is stopped or reduced, for instance when a limittemperature is not reached. Comparative data determined by simulation orempirically may additionally be taken into account in theadaptation/control. In the area of the cooled point one achieves maximumcooling and temperature difference. Starting therefrom, a certainflattening of the temperature distribution is observed during eachrotation. For the detection of the circumferential temperature profileone or more temperature measurement points are distributed around thecircumference of the deformation zone in the area of the longitudinalaxis position of the coolant action. A single measurement point isenough in the simplest case due to the rotation of the deformation zone.Suitable circumferential positions for a temperature measurement pointare, for instance, located opposite to the position of the coolantaction (after a rotation of the cooled point by about 180 degrees) ordirectly in front of this position (after a rotation between 300 degreesto 360 degrees). Suitable measuring devices are, e.g., infrared camerasor pyrometers.

As for the apparatus the above-indicated object, starting from anapparatus of the type mentioned at the outset, is achieved according tothe invention in that a coolant source is arranged around a deformationzone to dispense a coolant periodically to the deformation zone as soonas, due to the hollow-cylinder rotation, a circumferential positionhaving a comparatively small wall thickness passes the coolant source.

The apparatus according to embodiments of the invention is intended toexpand (inflate) at least one circumferential position having acomparatively small wall thickness along the longitudinal axis of thehollow cylinder, also shortly called “thin wall point,” into the tube toa smaller degree than neighboring wall portions. For this purpose, it isintended that a coolant acts on one or several previously determinedthin wall points, when this point passes into the deformation zone andits further deformation can be influenced by the action of a coolant.

For this purpose, the apparatus according to embodiments of theinvention is provided with a circumferentially preferably stationarysource for the discharge of coolant that is passed by the thin wallpoint periodically due to the rotation of the hollow cylinder. Thecoolant source is connected to a control device and is preferablyconfigured such that it dispenses coolant exactly once to thedeformation zone during each passage of the thin wall point during anaction period. The coolant amount applied during the action perioddepends on the degree of the wall thickness deviation to be correctedand is empirically determined and iteratively adapted in the simplestcase. The action period is typically within the range of a few seconds.It may be predetermined by the control device that with specificpassages of the thin wall point no coolant or a smaller amount ofcoolant is dispensed to the deformation zone.

In a preferred embodiment of the apparatus according to the inventionthe coolant source is configured to dispense a liquid coolant.

It comprises for instance a nozzle, a tube or an atomizer and is adaptedto transport the cooling liquid in the form of a jet, or as finedroplets having a diameter of less than 1 mm, to the intended actionarea of the deformation zone. When water, e.g., is used as a coolingliquid, it must be assumed, because of the comparatively low boilingpoint and the high temperatures in the deformation area, that hardly anyor little liquid, but at best vapor, passes onto the surface itself,wherein, however, heat is removed due to the evaporation from thesurroundings around the (theoretical) impact point and the impact pointis thereby cooled.

The coolant source is preferably movable together with the heater or amolding tool in the direction of the longitudinal axis of the hollowcylinder, but is stationary in circumferential direction. The action ofthe coolant is restricted to a circumferential section or to a circulararc around the deformation zone. The shorter this circular arc, the moreaccurately the coolant dispensed by the coolant source acts only on thethin wall point. In this respect, the circular arc around thedeformation zone in which the coolant is operative is preferably lessthan 30 angular degrees.

It has also proven to be useful when the coolant source is connected toa control device for the inner diameter, the outer diameter or the wallthickness of the wall of the tube and is configured to dispense a givencoolant amount in response to a control signal of the control device.

This embodiment is particularly also suited for the compensation ofseveral thin wall points distributed around the circumference, in thatthe coolant source can be activated by the control device during ahollow-cylinder rotation repeatedly for the discharge of coolant.

To be able to determine the temperature profile around the circumferenceof the deformation zone, one or several temperature measuring devicesare distributed in a particularly preferred embodiment of the apparatusaround the circumference of the deformation zone in the area of thelongitudinal axis position of the coolant action. Suitable measuringdevices are, for instance, infrared cameras or pyrometers.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown. In thedrawings:

FIG. 1 is a schematic, side view representation of an embodiment of anapparatus according to the invention for forming a hollow cylinder ofquartz glass into a quartz glass tube;

FIG. 2 is a schematic, sectional representation of the apparatus of FIG.1 showing additional constructional details;

FIG. 3 is an enlarged schematic representation view of the wallthickness profile of the hollow cylinder in the area of the deformationzone at the time when a coolant supply is switched on;

FIG. 4 is a view similar to FIG. 3 of the wall thickness profile in thearea of the deformation at the time when the coolant supply is switchedoff; and

FIG. 5 is a graphical diagram for explaining the influence of thecoolant supply on the wall thickness profile during a forming process.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows an apparatus for forming a hollow cylinder 2of quartz glass into a larger tube 22. The forming process comprisesseveral forming stages in which the respective initial hollow cylinderis formed, starting from an outer diameter of 300 mm, successively intothe desired larger tube 22 having an outer diameter of 960 mm and a wallthickness of 7.5 mm.

Holding tubes 3 are welded onto the ends of the hollow cylinder 2 ofquartz glass which is to be formed. The holding tubes 3 are clamped inchucks 4 of a horizontal glass lathe 5, which are synchronously rotatingabout the rotation axis 6. A burner carriage 21 (see FIG. 2), on which aplurality of burners are distributed in ring form around the outercircumference of the hollow cylinder 2, is moved from onehollow-cylinder end to the other end, thereby heating the hollowcylinder 2 of quartz glass zone by zone and around its entirecircumference. The burner carriage 21 is symbolized in FIG. 1 by adash-dotted circumferential line 20, which corresponds to the heatingzone; it is schematically shown in detail in FIG. 2. Via a gas inlet 9,the inner bore 7 of hollow cylinder 2 and larger tube 22 can be flushedwith gas, and a defined internal pressure can be set. Driven by thecentrifugal force and the internal pressure, the outer tube wall comesto rest on a molding of graphite 8, which is moved together with theburner carriage 21.

The graphite molding 8 is mounted on a slide 19 which is movable alongthe longitudinal axis 6. Moreover, a water jet tube 18 which is directedonto the deformation zone 14 between hollow cylinder 2 and tube 22 ismounted on the slide 19. The water jet tube 18 periodically produces afine water jet having a diameter of less than 5 mm. The water jet canimpinge in liquid form or in vapor form on the deformation zone 14.

The detail view of FIG. 2 shows the slide 19 with the water jet tube 18mounted thereon and the deformation zone 14 between hollow cylinder 2and tube 22. The water jet tube 18 is connected via a data and controlline 23 to a control device 17.

The burner carriage 21 moves along the initial hollow cylinder 2 fromthe right side to the left side, as shown by the directional arrow 13.The burner carriage 21 has mounted thereon in successive order twoburner rings 15 a, 15 b that are running in parallel around the rotationaxis 6 and serve to heat and soften the initial cylinder 2. The twoburner rings 15 a, 15 b are spaced apart in axial direction 6 by 50 mmand are adjustable in their heating capacity independently of eachother. Each of the burner rings 15 a, 15 b is formed of five gas burnersthat are evenly distributed around the longitudinal axis 6 of thecylinder, wherein, viewed in circumferential direction, the individualburners of the burner rows 15 a, 15 b are arranged offset from oneanother.

Due to the advance movement of the burner carriage 21 at a speed of 4cm/min, the hollow cylinder 2 while rotating about its longitudinal axis6 (which corresponds to the rotation axis) is heated continuously underthe action of the burner rings 15 a, 15 b to a high temperature of about2100° C. A lower heating capacity is here set in the rear burner ring 15b in comparison with the front burner ring 15 a.

The inner bore 7 may here be flushed with a gas, and a defined andcontrolled internal pressure of up to about 100 mbar can be set in theinner bore 7. A blow pressure of 15 mbar is applied in the embodiment.

The quartz glass is given such a low viscosity by the heating in theburner rings 15 a, 15 b that it deforms solely under the action ofcentrifugal force and internal pressure and without use of a moldingtool into the tube 22. The forming process is thus without tools. As asupport, the outer tube wall comes to rest on a molding 8 of graphite.

To measure the wall thickness, optical sensors 16 which are connected toa control device 17, including a wall thickness control, are arranged inthe area of the initial cylinder 2 and in the area of the drawn-offquartz glass tube 22. The sensors 16 are able to continuously produce awall thickness profile, while the tube strand is rotating, which profileis evaluated in the control device 17 such that the amount of wallone-sidedness (maximum value minus minimum value of the wall thickness)and the circumferential position of the minimum wall thickness (thinwall point) and the maximum wall thickness over the outer circumferenceare detected.

To measure the surface temperature in the area of the deformation zone14, a pyrometer 11 is directed onto a measurement point 12. The positionof the temperature measurement point 12 is positioned opposite to theimaginary impact point for the water jet from the water jet tube 18 ontothe deformation zone 14 (having an offset of about 180 degrees inrotation direction). The temperature profile around the circumference ofthe deformation zone 14 is thereby detected. This information issupplied via a data and control line (not shown) to the control device17 and used in addition to the temperature or wall thickness control.

FIG. 3 schematically shows the wall thickness profile of the hollowcylinder 2 in the area of the deformation zone 14 in a radialcross-section. The rotation direction around the longitudinal axis 6 isdesignated by the arrow 40, and the previously determinedcircumferential position of the thinnest hollow-cylinder wall isdesignated with the reference numeral 41. The control device 17 controlsthe water supply via the water jet tube 18, such that a water jet 44 issplashed briefly during passage of the previously determined thin wallpoint 41. The cooling water supply already sets in shortly before thethin wall point 41 reaches the circumferential position of the water jettube 18, and it ends shortly after the thin wall point 41 has passed thecircumferential position of the water jet tube 18, as shown in FIG. 4.

In this embodiment, the angle α between the circumferential position 42with incipient water jet 44 and thinnest wall point 41 is about 10degrees, and the angle β between the circumferential position 43,starting from which the water jet 44 is again switched off, and thethinnest wall point 41 is about 5 degrees. In response to thehollow-cylinder rotation, one thereby obtains a “pulsating”cooling-water discharge onto the deformation zone 14 via a circular arcof about 15 degrees, which surrounds the thinnest wall point 41. Thedischarge of cooling water is only carried out when the thinnest wallpoint 41 is positioned within this circular arc. When the thinnest wallpoint 41 leaves the circular arc, the discharge of cooling water isterminated.

The viscosity of the quartz glass around the thinnest wall point 41 isthereby locally increased, and the deformability of the glass mass isthereby reduced in this section of the deformation zone. The thinnestwall point 41 remains thicker than would be the case without the coolinglocally acting on it. Since the viscosity of the quartz glass showsexponential temperature dependence, temperature variations of a fewdegrees already have a noticeable effect on the viscosity.

When the hollow cylinder 2 is rotating with the initial outer diameterof 300 mm about the longitudinal axis 6 at a rotation speed of 30 rpm,this leads in the area of the deformation zone 14 (depending on thelocal circumference) to a tangential speed of more than 0.5 m/s. Aquartz glass strip having a width of 1 cm and a wall thickness of 1 cmthus shows a mass velocity of about 0.11 kg/s. To cool this strip by 1K, an energy conversion of about 150 J/s must be discharged (at aspecific thermal capacity of quartz glass of about 1.4 J/gK), whichcorresponds to a water amount of 0.06 g/s (on the assumption that theentire water amount evaporates).

Due to the pulsating periodic cooling-water discharge, the viscosity ofthe glass mass, which is rotating about the longitudinal axis 6 of thehollow cylinder, is influenced over the circumference of the deformationzone 14 such that, independently of the wall thickness profile of thehollow cylinder, one achieves a more uniform preforming in thedeformation zone.

In the case of several forming stages for producing the final tubediameter, it is enough when the wall thickness correction is carried outby periodic cooling-water discharge in the last forming stage. This isalso demonstrated by the diagram of FIG. 5, which shows the wallthickness profile of neighboring length sections of a quartz glass tubeafter the last forming stage (outer diameter of the initial cylinder2=320 mm, outer diameter of the final tube 22=440 mm, nominal wallthickness 4.7 mm). In the diagram, the wall thickness W is plotted (inmm) against the circumferential angle delta (in degrees). The initialcylinder 2 shows a thin wall point which extends with a uniform patternand at the same circumferential position (in FIG. 5 at about 160degrees) over its entire length. During the forming of the one lengthsection of the tube (curve A), the thin wall point was treated on thebasis of a periodic cooling-water discharge according to the invention.By comparison with the non-treated length section (curve B), the degreeof wall one-sidedness (siding), calculated as a maximum wall thicknessminus minimum wall thickness, could be reduced from 0.76 mm to 0.59 mmby use of the cooling-water measure in the last forming stage alone.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

We claim:
 1. A method for producing a tube of glass comprising thefollowing method steps: (a) providing a hollow cylinder of the glasshaving a bore defined by a wall, a wall thickness, and an initial outerwall diameter D₁; (b) heating and softening the hollow cylinder in azonewise manner while the hollow cylinder is rotating about a rotationaxis of the hollow cylinder and a heating zone is moved axially relativeto the rotation axis; (c) forming a deformation zone by radial expansionof a softened area of the hollow cylinder under action of at least oneof centrifugal force and internal overpressure applied in the hollowcylinder bore; and (d) continuously deforming the hollow cylinder to atube having a second outer diameter D₂ which is greater than initialdiameter D₁, wherein step (a) includes a step of determining acircumferential position at which the wall thickness is minimal around acircumference of the hollow cylinder, and wherein during heating andsoftening of the rotating hollow cylinder, a coolant is dispensed from acoolant source onto the deformation zone only when or predominantly whenthe circumferential position having the minimal wall thickness passesthe coolant source.
 2. The method according to claim 1, wherein a liquidis used as the coolant.
 3. The method according to claim 2, wherein theliquid is sprayed or splashed onto the deformation zone.
 4. The methodaccording to claim 2, wherein the liquid comprises water.
 5. The methodaccording to claim 2, wherein the liquid is changed in amount during acycle of the hollow-cylinder rotation.
 6. The method according to claim5, wherein the liquid amount during each rotation of the hollow cylinderabout its longitudinal axis is increased exactly once and is decreasedexactly once.
 7. The method according to claim 1, wherein a circular arcaround the deformation zone in which the coolant is operative is smallerthan 30 angular degrees.
 8. The method according to claim 1, wherein theinternal overpressure is set to less than 20 mbar.
 9. The methodaccording to claim 1, wherein the produced tube has an outer diameter D₂which is greater than the initial diameter D₁ by at least 40 mm.
 10. Themethod according to claim 1, further comprising a step of determining atemperature profile around a circumference of the deformation zone. 11.The method according to claim 1, wherein the internal overpressure isset to less than 10 mbar.
 12. The method according to claim 1, whereinthe outer diameter D₂ of the produced tube is greater than the initialdiameter D₁ by at least 70 mm.
 13. The method according to claim 1,wherein the outer diameter D₂ of the produced tube is greater than theinitial diameter D₁ by at least 100 mm.